This invention relates to a method of detecting a fault of an exhaust gas recirculation system which recirculates or returns part of the exhaust gas of an internal combustion engine to an intake passage of the same.
Conventionally, an exhaust gas recirculation system is known which recirculates part of the exhaust gas discharged from an internal combustion engine to the intake passage so as to decrease the amounts of noxious ingredients in the exhaust gas, such as NOx. The exhaust gas recirculation system includes an exhaust gas recirculation passage connecting the exhaust passage to the intake passage, an exhaust gas recirculation valve (hereinafter referred to as "EGR valve") arranged in the exhaust gas recirculation passage for opening/closing the same, and a control unit for controlling the operation of the EGR valve. The EGR valve is opened/closed in accordance with the operating conditions of the engine, such that a suitable amount of the exhaust gas flows back to the intake passage.
If, however, carbon etc. contained in the exhaust gas accumulates in the EGR valve of the exhaust gas recirculation system and decreases the opening of the passage, the necessary amount of exhaust gas cannot be introduced into the intake passage, thus degrading the emission characteristics of the engine. Such abnormality or fault of the exhaust gas recirculation system cannot usually be noticed by the driver unless it is detected by some fault detecting means.
Conventionally, A method is known in which the temperture of the recirculated exhaust gas is detected by a temperature sensor (hereinafter referred to as "EGR temperature sensor") arranged near the EGR valve either in communication with the exhaust gas recirculation passage or with a wall intervening therebetween when the exhaust gas recirculation system is in a condition in which it should be operated, thereby to detect a fault of the system. The fault detection of this method is based on the understanding that the temperature detected by the EGR temperature sensor differs greatly between the case where the EGR valve etc. operate normally and therefore a required amount of exhaust gas flows therethrough and the case where no or very little exhaust gas flows through the system due to abnormality of the EGR valve etc. Thus, the temperature difference is utilized for the detection of fault of the exhaust gas recirculation system. In this method, the exhaust gas recirculation system is determined to be defective when the temperature of the recirculated exhaust gas, detected by the EGR temperature sensor, is lower than a predetermined fault discrimination value.
However, the temperature of the recirculated exhaust gas varies largely in response to various conditions of air to be sucked into the engine, e.g., intake air temperature and atmospheric pressure. In the fault detecting method described above, therefore, if a single and fixed value is used as the fault discrimination value, it must be set at a low value in consideration of various engine operating conditions. And, if the fault detection is made using such a low fault-discriminating temperature, the exhaust gas recirculation system can erroneously be detected to be operating normally thought it actually is defective.
Further, when the engine is not yet completely warmed up immediately after the start-up, its operation is unstable and therefore, the temperature of the recirculated exhaust gas is varying unstably. While the engine is operating in such a condition, an accurate fault detection of the exhaust gas recirculation system cannot be made with the above method, possibly causing an erroneous detection.